Microalgae Grown in Photobioreactors for Mass Production of Biofuel

Need for Sustainable Energy The Price of Energy is Going UP Oil reserves are depleting World demand for energy is increasing Economic Stability/National Security Not good to have energy dependence on foreign countries Global Warming Fossil fuels release greenhouse gases

Why focus on Biodiesel? Ease of Implementation Diesel engines require little modification Many homes currently use oil for heat Less Fuel Would be Needed than Ethanol Has a 56.25% higher energy content Diesel engines are more efficient than spark-plug plug

Oil Yield Comparison Crop Oil yield (L/ha) Land area needed (M ha) a Percent of existing US cropping area a Corn 172 1540 846 Soybean 446 594 326 Canola 1190 223 122 Jatropha 1892 140 77 Coconut 2689 99 54 Oil palm Microalgae b Microalgae c 5950 136,900 58,700 45 2 4.5 a For meeting 50% of all transport fuel needs of the United States. b 70% oil (by wt) in biomass. c 30% oil (by wt) in biomass. 24 1.1 2.5

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Advantages of Algae High Quality Biodiesel Cold filter plugging point comparable to #2 diesel 2 No Phosphatides,, as in biodiesel from plants 3 Can capture CO 2 from exhaust streams Less Land is Required More biomass is obtained Contains higher concentrations of lipids than terrestrial plants

Oil Content of Some Microalgae 5 Microalga Oil content (% dry wt) Botryococcus braunii 25 75 Chlorella sp. 28 32 Crypthecodinium cohnii 20 Cylindrotheca sp. 16 37 Dunaliella primolecta 23 Isochrysis sp. 25 33 Monallanthus salina >20 Nannochloris sp. 20 35 Nannochloropsis sp. 31 68 Neochloris oleoabundans 35 54 Nitzschia sp. 45 47 Phaeodactylum tricornutum 20 30 Schizochytrium sp. 50 77 Tetraselmis sueica 15 23

Algae Production Methods Raceway Ponds Open system, used for production of algae for health food Photobioreactors Maximize algae growth with controlled conditions

Photobioreactor Variations

Maximize the Biomass Production Rate, g/l-d Equal to the product of the dilution rate and effluent biomass concentration. Defined as the ratio of the incoming flow rate to the reactor volume Dilution rate is equal to the specific growth rate at steady state

Specific Growth Rate Defined as the increase in cell mass per unit time per unit cell mass. May be determined empirically by: specific growth rate = [μ]= Ln (N2 / N1) / (t2 - t1) Where N1 and N2 = biomass at time1 (t1) and time2 (t2) respectively; Levasseur et al (1993).10 Its units are inverse hours. May be modeled as a function of solar irradiance and the maximum specific growth rate. Practical Model: μ = μ max *I av /(I k + I av ) Where Iav is the average irradiance inside the reactor and Ik is an organism-specific specific constant.

Design Considerations Effects of Solar Irradiance Solar inhibition Mass Transfer of Gases Through Fluid CO 2 supply and O 2 removal Cell Damage from Shear Stress For high flow rates Nutrient Addition, ph and Temperature Control

Solar Irradiance and Inhibition Dissolved Oxygen is directly related to photosynthetic activity. Photoinhibition causes decline in photosynthetic activity at midday.

Solution to Photoinhibition Increase cycle frequency of fluid between dark and light zones. Cycle frequency is increased by increasing the fluid velocity.

CO 2 Bubbling - Mass Transfer Carbon dioxide needs to be added continuously CO 2 can be consumed at a rate of 26 g CO 2 /m 3 -h Oxygen produced during photosynthesis needs to be removed. High oxygen concentration inhibits growth Photooxidation can damage cells Influence of the oxygen molar fraction in the injected gas on: (a) the steady-state biomass concentration; and (b) the photosynthetic activity (i.e. the volumetric oxygen generation rate) in indoor cultures. The dilution rate and the irradiance level were 0.025 h 1 and 300 μe m 2 s 1, respectively. 1

CO 2 Bubbling Effect of Fluid Velocity Superficial fluid velocity is related to gas velocity and bubble diameter Cycling frequency between light and dark zones is dependent on fluid velocity Shearing damage to cells results from increased radial velocity High radial velocity decreases length of micro-eddies So does increasing the tube diameter

Nutrient Addition, ph and Temperature Control Algae require nitrogen and phosphorous Ammonia is the preferred nitrogen source Conjunction with WWTP Optimal ph is between 7.5 and 8.5 Nutrient addition increases ph Sufficient CO 2 must be added to keep the ph from increasing too much Optimal temperature is between 20 and 30C Maintained with heat exchangers and cooling water Especially important at night to reduce losses due to respiration

Converting Algae to Fuel Transesterification Most common method of converting vegetable oil to biodiesel Requires the algal suspension first be harvested, dried, and pressed for oil

Converting Algae to Fuel Thermochemical Liquefaction Can be applied directly to algal suspension Uses high temperature and pressurized nitrogen to evaporate water CH 2 Cl 2 Catalyst Separates Biodiesel

Thermochemical Liquefaction of B. braunii Heating energy for liquefaction: 6.69 MJ/kg For a biomass concentration of 0.5 g/l produced from raceway ponds Heating value of oil produced: 45.9 MJ/kg Concentration in photobioreactors: 6.6g/L Much less energy required on kg basis

Achieved Rates of Productivity for P. tricornutum Volume L U L m/s D, h -1 I wm C b, g/l P b, g/l-d ν, Hz Source 220 0.3 0.025 2366 6.6 1.66 0.684 16 220 0.3 0.04 2319 4.4 1.76 0.0628 16 220 0.3 0.04 2860 5.1 2.04 0.0638 16 220 0.3 0.04 1211 2.7 1.08 0.651 16 200 0.5 0.05 1289 2.38 1.19-11

Energy Yield Maximum tube length = 80 m Maximum tube diameter = 0.1 m So maximum volume of single reactor = 628.3 L = 0.6283 m 3 Assume oil production rate = 2 g/l-d * 50% oil content = 1 g/l-d = 1 kg/m 3 -d If 270 tubes can fit on one acre, than 62,000 kg of oil/acre could be produced. Or (density = 0.864 kg/l) 71,759 L = 19,136 gal/acre

Wow!! Almost 20,000 gallons/acre of oil that may potentially be produced with current technology With thermochemical liquefaction, the energy required to extract oil is minimal Main Constraint: Huge Capital Investment reactor tubes, water pumps, gas pumps, autoclave for liquefaction. Economies of Scale How long before oil production pays for infrastructure investment?